Incidence and outcomes of acute kidney injury after cardiac surgery using either criteria of the RIFLE classification

The present retrospective monocentric study includes 443 patients undergoing elective or emergent cardiac surgery, and investigates the incidence and outcomes of postoperative AKI, as well as the pre- and per-operative risk factors for AKI. The incidence and outcomes of AKI vary according to its definition. In the present study, we first used the conventional 2004 RIFLE classification which is based on the elevation of SCr and the decrease of UO [6]. This classification has been approved for cardiac surgery [1, 7‐9]. In an original approach, we compared AKI incidence and outcomes according to SCr or UO criteria. We observed a 7-day incidence of AKI reaching 50 % by taking into account both RIFLE criteria. This percentage of postoperative AKI is rather high, as compared to previous reports. This may be partly explained by the use of all daily values of SCr and UO for each patient over the 7-day period following surgery. In a similar retrospective study, D’Onofrio et al. reported an incidence of postoperative AKI of 23.5 % [7]. These authors only tracked the single variation in SCr during the length of hospitalisation. Conversely, by using the complete RIFLE classification, Hobson et al. reported an incidence of AKI reaching 43.0 % after cardiac surgery, which corresponds with our observations [1]. Interestingly, each component of RIFLE, i.e. SCr or UO, differentially contributes to AKI incidence and outcomes, as previously illustrated by McIlroy et al. [10] and Wlodzimirow, et al. [11]. In our series, the elevation of SCr was observed in 9.7 %, whereas a decreased UO was noted in 40.2 %. Such a 4-fold increase in AKI incidence when the UO criterion is taken into account provides cause for concern regarding its validity and the true significance of diagnosed AKI_UO. To apply the SCr criteria of RIFLE, information on prior renal function is needed. When a pre-ICU admission SCr is not available, RIFLE group suggests that the baseline SCr be estimated from the MDRD formula [3]. Still, mathematical estimations of baseline SCr have been associated with both over- or under-diagnosis of AKI [12, 13]. Here, baseline SCr systematically corresponded with its value 7 days before surgery. Patients developing AKI_SCr showed a significantly higher preoperative morbidity, including hypertension, COPD, CKD and anaemia, than AKI_UO patients (Table 2). Moreover, 1-year mortality rate was significantly higher in the case of AKI_SCr compared to AKI_UO (Fig. 1). Systematic reviews investigating the validity of RIFLE classification in general ICU patients concluded that the mortality risk was higher in the absence of UO criterion in the AKI definition [14, 15]. Still, discarding the UO criterion has been suggested to delay the diagnosis of AKI, thereby increasing the mortality in general ICU patients [11]. In the particular settings of patients undergoing cardiac surgery, oliguria has also been associated with an increased incidence of AKI, but with a reduced discriminant utility for mortality in comparison to elevated SCr [10, 16]. As a whole, the inclusion of UO consensus criterion causes a questionable impact on the overestimated incidence of AKI after cardiac surgery.

The pathophysiology of postoperative AKI is multifactorial [17]. In our cohort, patients developing post surgery AKI were older and more fragile according to the Parsonnet and EuroSCORE1 scores, and presented with a higher BMI. Furthermore, increased BMI is significantly associated with DM (29.0 [25.8 - 31.6] vs 25.7 [23.1 - 28.3], p < 0.001) and hypertension (26.4 [23.9 - 29.9] vs 25.7 [23.0 - 27.8], p < 0.02). These risk factors were further highlighted by our multivariate model (Table 4). Pre-operative CKD reflected by a lower GFR before surgery was also associated with an increased risk for AKI. We estimated the pre-operative GFR using the MDRD formula based on serum creatinine concentration 7 days before surgery [18]. Most patients (76.7 %) showed a GFR within the normal range, i.e. > 60 ml/min. Patients presenting with impaired kidney function before surgery were significantly older (74 [68 - 78] years) than patients with GFR > 60 mL/min (66 [59 - 74] years, p < 0.001). This represents a selection bias. Indeed, given the impact of age in the multivariate logistical regression Parsonnet model, the predicted mortality rate at 30 days post surgery was higher in the CKD group compared to the rest of the cohort. The same pattern was observed with the logistical model EuroSCORE1 [5]. Similarly, we observed a lower 1-year survival probability in patients with a pre-operative GFR <60 mL/min [19].

The type and complexity of cardiac surgery was identified as an independent predictor of post-operative AKI upon our multivariate model (Table 3). At our institution, the choice between Off-pump and On-pump procedures essentially depends on the technical feasibility of myocardial revascularisation on the basis of coronarography images. In the present cohort, the types of surgeries, with or without CPB, are not distributed homogeneously, with more patients with normal renal function (15.3 %) being operated on using the Off-pump technique compared to CKD patients (5.83 %). Still, we did not observe a significantly higher incidence of AKI in On-pump versus Off-pump groups, in contrast to previous reports [20‐22]. Patients with normal pre-operative renal function were mostly operated for isolated CABG, whereas only a minority (10.0 %) underwent combined surgery. Conversely, the distribution of surgery types appears more balanced in the CKD group, with 39.8 % of patients operated for CABG or isolated valve and 20.4 % for combined surgery. Such an imbalance between non-CKD and CKD patients may represent an additional selection bias in our analysis since the type of surgery influences the duration of aortic clamping and CPB, which in turn may favor AKI development.

In the present cohort, post-operative AKI was associated with a significantly longer stay in the ICU, with no impact on the entire in-hospital period. Moreover, the ICU period significantly increased with the degree of AKI severity based on the RIFLE stages. Elevation of SCr appeared to be associated with a prolonged ICU stay in comparison to UO, although this observation did not reach statistical significance (p = 0.07). Requirement for RRT was associated with a dramatic increase in the in-hospital mortality (75.5 %), as was previously reported [23‐25].

The present study has several limitations, including its retrospective and monocentric design with a limited number of patients. Secondly, the majority of surgical interventions were planned (97.3 %), which limits the extension of the observations to emergency surgeries. Indeed, the need for nephrotoxic contrast medium at the time of preoperative coronarography may precipitate AKI development in cases of early cardiac surgery [26, 27]. Thirdly, the priming solution of CPB circuit most often consists of Volulyte 6 % (4th generation balanced hydroxyethyl starch). Recently, the use of such a starch has been associated with an increased incidence of AKI, as well as with postoperative bleeding [28]. Volulyte 6 % solution was chosen for CPB priming in order to reduce the risk for anaphylactoid shocks observed with some colloids or gelatins. New generation starches appear to be less nephrotoxic, provided that the perfusion is not extended beyond 24 h [29‐31]. The optimal priming solution for CPB circuits remains a matter of debate.